This invention relates generally to vacuum arc melting and, more particularly, to vacuum arc melting furnace operation in the production of pure zirconium and specifically to crucible coatings used in the crucible of vacuum arc melting furnaces which are used in the production of pure zirconium.
Vacuum arc melting of electron beam welded compacts of zirconium metal sponge, chips or chunks have been employed conventionally to produce ingots of pure zirconium. The welded together compacts form an elongated cylinder which is used as the vertical electrode which is lowered into a copper crucible, which normally contains a water jacket for cooling and the melt is produced by the application of sufficient electrical power to completely arc melt the zirconium to form a melt which cools into an ingot conforming to the shape of the copper crucible.
In the production of zirconium ingots in vacuum arc melting furnaces, the cooled cylindrically shaped ingot is removed from the crucible, the ends squared, and the ingot turned on a lathe to remove the surface that has contacted the crucible. The ingot surface is machined to eliminate folds, laps and unevenness in the physical surface of the ingot as well as the hydrogen which alters the chemistry and physical properties of the zirconium metal. This later step is both time-consuming and costly. It has been necessary however, in the past, to make the surface smooth and to remove surface impurities, including zirconium which has been embrittled by hydrogen at and near the surface of the ingot. Typically there will be adsorbed in the ingot surface as much as 35 to 50 parts per million hydrogen due to the conditions encountered in the vacuum arc furnace. The hydrogen level should however, be less than 5 parts per million in order to eliminate cracking at the surface of the ingot in the forging operation. Both lower hydrogen levels and a smoother surface are necessary in order to eliminate the scalping operation and prevent cracking during forging.
Zirconium and many other reactive metals absorb hydrogen in large amounts to form hydrides..sup.(1) The absorption of hydrogen into zirconium and/or these other metals alters their physical properties from soft, ductile, malleable metals to hard brittle intermetallic compounds. This property of absorbing hydrogen has been known and used for some time to make powders of otherwise ductile metals. Hydrogen may be absorbed or desorbed in a range of temperatures from 250.degree. C. up to 850.degree. C. Desorption is usually accomplished at the higher temperatures while evacuating the furnace by means of vacuum pumps or a combination of a sweep gas and vacuum pumping. A reactive metal may be hydrided by heating under hydrogen atmosphere at greater than 250.degree. C. but less than 600.degree. C. and then cooling to room temperature. The hydrided metal is the crushed to powder and the powder heated in vacuum to dehydride and return the metal to its ductile form. Very small amounts of hydrogen can promote cracking in zirconium when undergoing normal fabrication techniques. FNT .sup.(1) Metal Hydrides, William M. Mueller, James P. Blackledge, George. Libowitz. Academic Press, N.Y. and London 1968
Generally less than 5 parts per million is the preferred level to avoid fabrication problems such as cracking in zirconium.
In a process which produces 18,000 lb. ingots, the squaring and surface removal operation may require removal of as much as 1,000 lbs. of zirconium metal. This is a significant amount of scrap which needs to be recycled and which would be desirable to avoid or eliminate.
It is, therefore, an objective of the present invention to provide a method of making a coating composition and a coating composition which does not introduce impurities into the surface of a zirconium ingot and helps in the prevention of hydrogen absorption by providing an environment which promotes desorption to less than five parts per million.
It is a further objective of the present invention to provide a new method of making zirconium ingots by melting compacts of zirconium metal in a vacuum arc furnace which ingots exhibit less hydrogen embrittlement at the surface, thereby requiring less removal of surface metal to obtain a pure zirconium ingot with a sound surface.
It is a further object of the present invention to provide a vacuum arc furnace crucible with a coating which has better heat insulating properties than prior coatings, thereby producing an improved method of making zirconium ingots by enabling higher melt rates to be achieved during the production of zirconium ingots from zirconium compacts in a vacuum arc melting furnace.